PlantUMLLanguage specificationUse Case Diagram
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用例图

Let's have few examples :

Note that you can disable the shadowing using the skinparam shadowing false command.

用例

用例用圆括号括起来。

也可以用关键字usecase来定义用例。 还可以用关键字as定义一个别名，这个别名可以在以后定义关系的时候使用。 

@startuml

(First usecase)
(Another usecase) as (UC2)
usecase UC3
usecase (Last\nusecase) as UC4

@enduml

角色

角色用两个冒号包裹起来。

也可以用actor关键字来定义角色。 还可以用关键字as来定义一个别名，这个别名可以在以后定义关系的时候使用。

后面我们会看到角色的定义是可选的。 

@startuml

:First Actor:
:Another\nactor: as Men2
actor Men3
actor :Last actor: as Men4

@enduml

用例描述

如果想定义跨越多行的用例描述，可以用双引号将其裹起来。

还可以使用这些分隔符：--..==__。 并且还可以在分隔符中间放置标题。 

@startuml

usecase UC1 as "You can use
several lines to define your usecase.
You can also use separators.
--
Several separators are possible.
==
And you can add titles:
..Conclusion..
This allows large description."

@enduml

Use package

You can use packages to group actors or use cases. 

@startuml
left to right direction
actor "Food Critic" as fc
package Restaurant {
  usecase "Eat Food" as UC1
  usecase "Pay for Food" as UC2
  usecase "Drink" as UC3
}
fc --> UC1
fc --> UC2
fc --> UC3
@enduml

You can use rectangle to change the display of the package. 

@startuml
left to right direction
actor "Food Critic" as fc
rectangle Restaurant {
  usecase "Eat Food" as UC1
  usecase "Pay for Food" as UC2
  usecase "Drink" as UC3
}
fc --> UC1
fc --> UC2
fc --> UC3
@enduml

基础示例

用箭头-->连接角色和用例。

横杠-越多，箭头越长。 通过在箭头定义的后面加一个冒号及文字的方式来添加标签。

在这个例子中，User并没有定义，而是直接拿来当做一个角色使用。 

@startuml

User -> (Start)
User --> (Use the application) : A small label

:Main Admin: ---> (Use the application) : This is\nyet another\nlabel

@enduml

继承

如果一个角色或者用例继承于另一个，那么可以用<|--符号表示。 

@startuml
:Main Admin: as Admin
(Use the application) as (Use)

User <|-- Admin
(Start) <|-- (Use)

@enduml

使用注释

可以用note left of , note right of , note top of , note bottom of等关键字给一个对象添加注释。

注释还可以通过note关键字来定义，然后用..连接其他对象。 

@startuml
:Main Admin: as Admin
(Use the application) as (Use)

User -> (Start)
User --> (Use)

Admin ---> (Use)

note right of Admin : This is an example.

note right of (Use)
  A note can also
  be on several lines
end note

note "This note is connected\nto several objects." as N2
(Start) .. N2
N2 .. (Use)
@enduml

构造类型

用 << 和 >> 来定义角色或者用例的构造类型。 

@startuml
User << Human >>
:Main Database: as MySql << Application >>
(Start) << One Shot >>
(Use the application) as (Use) << Main >>

User -> (Start)
User --> (Use)

MySql --> (Use)

@enduml

改变箭头方向

默认连接是竖直方向的，用--表示，可以用一个横杠或点来表示水平连接。 

@startuml
:user: --> (Use case 1)
:user: -> (Use case 2)
@enduml

也可以通过翻转箭头来改变方向。 

@startuml
(Use case 1) <.. :user:
(Use case 2) <- :user:
@enduml

还可以通过给箭头添加left, right, up或down等关键字来改变方向。 

@startuml
:user: -left-> (dummyLeft)
:user: -right-> (dummyRight)
:user: -up-> (dummyUp)
:user: -down-> (dummyDown)
@enduml

这些方向关键字也可以只是用首字母或者前两个字母的缩写来代替。

但是请注意，这样的缩写不要乱用，Graphviz不喜欢这样。

分割图示

用newpage关键字将图示分解为多个页面。 

@startuml
:actor1: --> (Usecase1)
newpage
:actor2: --> (Usecase2)
@enduml

从左向右方向

默认从上往下构建图示。 

@startuml
'default
top to bottom direction
user1 --> (Usecase 1)
user2 --> (Usecase 2)

@enduml

你可以用left to right direction命令改变图示方向。 

@startuml

left to right direction
user1 --> (Usecase 1)
user2 --> (Usecase 2)

@enduml

显示参数

用skinparam改变字体和颜色。

可以在如下场景中使用：

你也可以给构造的角色和用例指定特殊颜色和字体。 

@startuml
skinparam handwritten true

skinparam usecase {
BackgroundColor DarkSeaGreen
BorderColor DarkSlateGray

BackgroundColor<< Main >> YellowGreen
BorderColor<< Main >> YellowGreen

ArrowColor Olive
ActorBorderColor black
ActorFontName Courier

ActorBackgroundColor<< Human >> Gold
}

User << Human >>
:Main Database: as MySql << Application >>
(Start) << One Shot >>
(Use the application) as (Use) << Main >>

User -> (Start)
User --> (Use)

MySql --> (Use)

@enduml

一个完整的例子

@startuml
left to right direction
skinparam packageStyle rectangle
actor customer
actor clerk
rectangle checkout {
  customer -- (checkout)
  (checkout) .> (payment) : include
  (help) .> (checkout) : extends
  (checkout) -- clerk
}
@enduml